"In January 2003 we enjoyed outstanding
views of these clouds above the southern hemisphere," said space
station astronaut Don Pettit during a NASA TV broadcast in January
2003."We routinely see them when we're flying over Australia and
the tip of South America."

Sky watchers on Earth have seen them,
too, glowing in the night sky after sunset, although the view from
Earth-orbit is better. Pettit estimated the height of the noctilucent
clouds he saw at 80 to 100 km ... "literally on the fringes of space."

"Noctilucent clouds are a relatively
new phenomenon," says Gary Thomas, a professor at the University
of Colorado who studies NLCs. "They were first seen in 1885" about
two years after the powerful eruption of Krakatoa in Indonesia,
which hurled plumes of ash as high as 80 km into Earth's atmosphere.

Ash from the volcano caused such splendid
sunsets that evening sky watching became a popular worldwide pastime.
One sky watcher in particular, a Briton named T. W. Backhouse, noticed
something odd. He stayed outside after the sun had set and, on some
nights, saw wispy filaments glowing electric blue against the black
sky. Noctilucent clouds. Scientists of the day figured the clouds
were some curious manifestation of volcanic ash.

Eventually the ash settled and the
vivid sunsets of Krakatoa faded. Yet the noctilucent clouds remained.
"It's puzzling," says Thomas. "Noctilucent clouds have not only
persisted, but also spread." A century ago the clouds were confined
to latitudes above 50o; you had to go to places like
Scandinavia, Russia and Britain to see them. In recent years they
have been sighted as far south as Utah and Colorado.

Noctilucent
clouds over Finland. The orange hues near the horizon are
ordinary sunset colours, notes Gary Thomas. NLCs, on the
other hand, are usually "luminous blue-white or sometimes
just pale white," he says.

Astronaut Don Pettit is a long-time
noctilucent cloud-watcher. As a staff scientist at the Los Alamos
National Laboratory between 1984 and 1996, he studied noctilucent
clouds seeded by high-flying sounding rockets. "Seeing these kinds
of clouds [from space] ... is certainly a joy for us on the ISS,"
he said on NASA TV.

"Although NLCs look like they're in
space," continues Thomas, "they're really inside Earth's atmosphere,
in a layer called the mesosphere ranging from 50 to 85 km high."
The mesosphere is not only very cold (-125 C), but also very dry - "one
hundred million times dryer than air from the Sahara desert." Nevertheless,
NLCs are made of water. The clouds consist of tiny ice crystals
about the size of particles in cigarette smoke. Sunlight scattered
by these crystals gives the clouds their characteristic blue colour.

How ice crystals form in the arid mesosphere
is the essential mystery of noctilucent clouds.

Photo credit: Don Pettit and NASA TV.

Another
noctilucent cloud seen from the ISS. Earth's horizon has
been deliberately overexposed to reveal the faint cloud
tops."That little diaphanous line you see paralleling Earth's
horizon is an NLC," said Pettit.

Ice crystals in clouds
need two things to grow: water molecules and something for those
molecules to stick to - dust, for example. Water gathering on dust
to form droplets or ice crystals is a process called nucleation.
It happens all the time in ordinary clouds.

Ordinary clouds, which are relatively
close to Earth, get their dust from sources like desert wind storms.
It's hard to waft wind-blown dust all the way up to the mesosphere,
however. "Krakatoa may have seeded the mesosphere with dust in 1883,
but that doesn't explain the clouds we see now," notes Thomas. "Perhaps,"
he speculates, "the source is space itself." Every day Earth sweeps
up tons of meteoroids - tiny bits of debris from comets and asteroids.
Most are just the right size to seed noctilucent clouds.

The source of water vapour is less
controversial. "Upwelling winds in the summertime carry water vapour
from the moist lower atmosphere toward the mesosphere," says Thomas.
This is why NLCs appear during summer, not winter.

One reason for the recent spread of
noctilucent clouds might be global warming. "Extreme cold is required
to form ice in a dry environment like the mesosphere," says Thomas.
Ironically, global warming helps. While greenhouse gases warm Earth's
surface, they actually lower temperatures in the high atmosphere.
Thomas notes that noctilucent clouds were first spotted during the
Industrial Revolution - a time of rising greenhouse gas production.

Are NLCs a thermometer for climate
change? An unusual sign of meteoroids? Or both? "So much about these
clouds is speculative," says Thomas.

A NASA spacecraft scheduled for launch
in 2006 should provide some answers. The Aeronomy of Ice in the
Mesosphere satellite, or AIM for short, will orbit Earth at
an altitude of 550 km. Although it's a small satellite, says Thomas,
there are many sensors on board. AIM will take wide angle photos
of NLCs, measure their temperatures and chemical abundances, monitor
dusty aerosols, and count meteoroids raining down on Earth. "For
the first time we'll be able to monitor all the crucial factors
at once."

Meanwhile, all we can
do is wait... and watch. There's never been a better time to see
noctilucent clouds. "During the summer months, look west perhaps
30 minutes to an hour after sunset when the Sun has dipped 6o
to 16o below the horizon," advises Thomas. If you see
luminous blue-white tendrils spreading across the sky, you've probably
spotted an NLC. Observing sites north of 40o latitude
are favoured.

One more thing: don't forget your camera.
According to astronaut Don Pettit, "you can never have too many
pictures of noctilucent clouds."

Please note: Astronaut Don Pettit's remarks and his pictures of NLCs that appear
in this story were first broadcast on NASA TV in January 2003.